A chromatin bound enzyme, poly(ADP-ribose) synthetase, synthesizes chains of ADP-ribose from substrate, NAD, and transfers these onto a variety of chromosomal proteins. This transfer markedly alters the overall charge of the so-modified proteins and consequently is believed to be involved in alterations of chromatin structure. Poly(ADP-ribose) synthetase on chromatin is activated on chromatin by perturbing chromatin structure such as by chromatin condensing agents or by the introduction of DNA chain breaks. If this appears that poly(ADP-ribose) synthesis is not the inducer of chromatin structural alterations but rather is an intermediate in this process. One proposed role of poly(ADP-ribose) synthetase, DNA strand break repair, is now found to be less likely. Heat shocked drosophila melanogaster cells loose all detectable poly(ADP-ribose) synthetase activity but are still capable of repairing gamma-ray induced DNA strand breaks.